1. Field of the Invention
This invention relates to a process for separating uranium isotopes by oxidation-reduced chromatography. More particularly, the present invention is concerned with a process for separating uranium isotopes by oxidation-reduction chromatography in which the separation of uranium isotopes and the regeneration of the oxidizing agent and reducing agent, both deactivated by the redox reaction during the separation of uranium isotopes, are simultaneously performed in a single column with great advantages.
2. Description of the Relevant Art
It is known that uranium isotopes can be separated by oxidation-reduction chromatography which comprises developing a uranium adsorption zone formed in a column of an anion exchanger while oxidizing the uranium adsorption zone at its front region and reducing the uranium adsorption zone at its rear region (see, for example, U.S. Pat. No. 4,112,045).
According to the method of U.S. Pat. No. 4,112,045, a solution containing an oxidizing agent which is capable of oxidizing uranium (IV) to uranium (VI) is supplied to a developing column packed with an anion exchanger to form an oxidizing agent adsorption zone. Then, a uranium isotope solution containing uranium (IV) is supplied to the column to convert a portion of the ozidizing agent adsorption zone to an adsorbed uranium (VI) zone. Thereafter, a solution containing a reducing agent which is capable of reducing uranium (VI) to uranium (IV) is supplied to the column to develop the adsorbed uranium (VI) zone while forming a reducing agent zone in rear of the adsorbed uranium (VI) zone, thereby causing the uranium (VI) adsorbed on the anion exchanger to be eluted in the form of uranium (IV). In the development of the adsorbed uranium (VI) zone, a boundary is formed between the oxidizing agent zone and the uranium (VI) zone, and a uranium solution recovered from the vicinity of this boundary has a high concentration of .sup.238 U. Also, another boundary is formed between the uranium (VI) zone and the reducing agent zone, and a uranium solution recovered from the vicinity of this boundary has a high concentration of .sup.235 U.
In the above-mentioned oxidation-reduction chromatography, a deactivated oxidizing agent and a deactivated reducing agent are eluted as a mixture thereof from the developing column. In this connection, there has been proposed a method for regenerating a deactivated oxidizing agent and a deactivated reducing agent contained in the eluate effluent from the developing column and for reusing them (see U.S. Pat. No. 4,202,860). According to the method, the eluate containing a deactivated oxidizing agent and a deactivated reducing agent is subjected to oxidization treatment to regenerate the deactivated oxidizing agent. The regenerated oxidizing agent is separated from the eluate using an anion exchanger. Then, the resulting eluate containing the deactivated reducing agent is subjected to reduction treatment to regenerate the deactivated reducing agent. The activated oxidizing agent and the activated reducing agent thus obtained are reused for further separation of uranium isotopes. In this method, the regeneration of the deactivaged oxidizing agent and the deactivated reducing agent are carried out outside the developing column by an oxidation-reduction reaction using oxygen and hydrogen, respectively, or by an electrolytic oxidation-reduction reaction.
The above-mentioned method disclosed in U.S. Pat. No. 4,202,860 is advantageous in that the regenerated oxidizing agent and the regenerated reducing agent can be used for further separation of uranium isotopes. However, the method has various disadvantages. Specifically, in the method, the separation of uranium isotopes in a single column (hereinafter after referred to as "single column separation method") is effected as follows. When a uranium adsorption zone has reached the bottom of a column packed with an anion exchanger, the separated uranium isotopes are fractionally collected, while a solution containing the deactivated oxidizing agent and the deactivated reducing agent which has been eluted and collected during the development of the uranium adsorption zone in the column is oxidized outside the column to regenerate the deactivated oxidizing agent. After completion of the collection of the separated uranium isotopes in fractions, the solution which has been subjected to oxidation treatment is supplied to the column. In the column, the regenerated oxidizing agent alone is adsorbed on the anion exchanger and the deactivated reducing agent is eluted. The eluted deactivated reducing agent is subjected to reduction treatment outside the column to regenerate the deactivated reducing agent. After completion of the adsorption of the regenerated oxidizing agent on the anion exchanger, the fractions of uranium isotopes are returned to the column to form a uranium adsorption zone. Subsequently, a solution containing the regenerated reducing agent is supplied to the column in order to further continue the development of the uranium adsorption zone. As is apparent from the foregoing, in the method of U.S. Pat. No. 4,202,860, the eluate containing the separated uranium isotopes effluent from the bottom of the column cannot be immediately returned to the column, but should be fractionally collected and separately reservoired prior to returning to the column until the adsorption of the regenerated oxidizing agent on the anion exchanger is completed. The fractional collection and separate reservation of the elute is not only troublesome but also results in partial mixing of the separated uranium isotopes, leading to a poor separation efficiency per unit time.
In order to eliminate the above-mentioned drawback accompanying the single column separation method in U.S. Pat. No. 4,202,860, there has been proposed a method of effecting the uranium isotope separation using two or more developing columns (hereinafter referred to as "multiple column separation method"). In the method, while the development of a uranium adsorption zone is performed in one developing column or two or more developing columns, an eluate containing the deactivated oxidizing agent and the deactivated reducing agent is subjected to oxidation treatment outside the column to regenerate the deactivated oxidizing agent. The treated eluate is supplied to at least one of the remaining developing columns where the development has not been effected, thereby forming an oxidizing agent zone while eluting the deactivated reducing agent. The deactivated reducing agent is subjected to reduction treatment outside the column to regenerate the deactivated reducing agent. The uranium adsorption zone which has been subjected to the separation of uranium isotopes in one developing column is transferred to the developing column where an oxidizing agent zone has been formed, thereby forming an uranium adsorption zone. Then, the regenerated reducing agent is supplied to the column in which the uranium adsorption zone has been formed. Thus, a uranium adsorption zone which has been developed in one column is immediately transferred to another developing column where an oxidizing agent zone has already been formed without the necessity of fractionally collecting and separately reservoiring the uranium eluate. However, the above-mentioned multiple column development method is disadvantageous in that the construction cost of two or more developing columns packed with an anion exchanger is high as compared with that of a single developing column and that the number of valves such as switchover valves to be used in the multiple column separation method is far larger than that to be used in the single column separation method, causing a danger of occurrence of valve trouble which leads to necessity of frequent shut-down of the operation due to the leakage of a liquid from the valves.
Further, in both the above-mentioned single column method and multiple column method, the whole of the deactivated oxidizing agent and the whole of the deactivated reducing agent are regenerated outside the column by means of oxygen and hydrogen, respectively, or by an electrolytic oxidization-reduction reaction. Therefore, these methods require a large amount of oxygen and hydrogen, or a large amount of electric power to regenerate the deactivated oxidizing agent and the deactivated reducing agent.
Therefore, both the above-mentioned methods, single column separation method and multiple column separation method, are disadvantageous from the commerical point of view.